1. Field of the Invention
The present invention relates to improvement in an electric power steering apparatus comprising a torque sensor which detects a steering torque applied to a steering wheel and a steering assist motor which is driven to rotate based on the steering torque detected by the torque sensor.
2. Description of Related Art
An electric power steering is mounted on a vehicle so as to assist steering of the vehicle. The electric power steering apparatus has a steering mechanism which is connected with a steering wheel via a steering shaft. The steering mechanism is provided with a torque sensor for detecting a steering torque applied to the steering wheel and a motor for assisting operations of the steering mechanism. As the motor is driven in accordance with the steering torque detected by the torque sensor, a steering force which a driver needs to apply to the steering wheel is reduced.
In recent years, a brushless motor is frequently used as a steering assist motor for an electric power steering apparatus. The brushless motor comprises a rotor having a permanent magnet. The rotor is rotated as a circuit generates a rotating magnetic field on a stator in a controlled manner based on a detected signal corresponding to a position of the rotating rotor. A brushless motor without a brush does not generate mechanical or electric noise. Consequently, the brushless motor is widely used for apparatuses which require a motor with high performance in rotations and a long life, such as a cylinder in a VTR, a capstan in a cassette deck, a flexible disk drive unit and a CD player.
FIG. 1 is a transverse sectional view showing the structure of a rotor of a brushless motor configured as an SPM (Surface Permanent Magnet) motor which is used for a conventional electric power steering apparatus. The rotor has a rotor core 3 consisting of a plurality of laminated electromagnetic thin steel plates. Disposed through a center portion of the rotor core 3 is a shaft 4. The rotor further has four permanent magnets 1, 2, 1 and 2 which are attached to the rotor core 3 along a peripheral portion of the rotor core 3 in a manner that each magnet has a polarity different from that of an adjacent magnet. Each axial length of the four permanent magnets 1, 2, 1 and 2 is approximately equal to the length of the rotor core 3 in a direction of the lamination.
In the SPM motor, there is the possibility that one of the permanent magnets 1 and 2 breaks or comes off and scatters causing the motor to be locked. Consequently, to prevent scattering, the permanent magnets 1 and 2 at the peripheral portion of the rotor core 3 are covered with a cylinder 5 (a can) which is made of a thin stainless steel plate of non-magnetic material.
A motor torque of the brushless motor configured as an SPM motor has an output characteristic as shown in FIG. 2, where an output (=torque * RPM (Revolutions Per Minute)) suddenly decreases in a high-speed rotation area. Consequently, when a brushless motor configured as an SPM motor is used as a steering assist motor for an electric power steering apparatus, a steer-assisting force cannot be obtained in high-speed steering and the steering operation becomes hard.